Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session C42: Electronic and Thermal Transport Properties of GrapheneLive
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Sponsoring Units: DCMP Chair: Di Chen, Peking University |
Monday, March 15, 2021 3:00PM - 3:12PM Live |
C42.00001: Dynamic response functions of two-dimensional Dirac fermions with screened Coulomb and short-range interactions Megha Agarwal, Eugene G Mishchenko We consider a screened Coulomb interaction between electrons in graphene and determine their dynamic response functions, such as a longitudinal and a transverse electric conductivity and a polarization function and compare them to the corresponding quantities in the short-range interaction model. The calculations are performed to all orders for short-range interaction by taking into account the self-energy renormalization of the electron velocity and using a ladder approximation to account for the vertex corrections, ensuring that the Ward identity (charge conservation law) is satisfied. Our findings predict a resonant response of interacting electron-hole pairs at a particular frequency below the threshold qv=ω and further predict an instability for sufficiently strong interactions. |
Monday, March 15, 2021 3:12PM - 3:24PM Live |
C42.00002: Spin-valley collective modes of the electron liquid in graphene Zachary Raines, Vladimir Falko, Leonid Glazman We develop the theory of collective modes supported by a Fermi liquid of electrons in pristine graphene. Under reasonable assumptions regarding the electron-electron interaction, all the modes but the plasmon are over-damped at finite and zero temperature. In addition to the SU(2) symmetric spin mode, these include also the valley imbalance modes obeying a U(1) symmetry, and an SU(2) symmetric valley spin imbalance mode. We derive the interactions and diffusion constants characterizing the over-damped modes. The corresponding relaxation rates set fundamental constraints on graphene valley- and spintronics applications. |
Monday, March 15, 2021 3:24PM - 3:36PM Live |
C42.00003: Mechanical and Electronic Properties of Graphene under Periodically Modulated Strain Riju Banerjee, Viet-Hung Nguyen, Malgorzata Kowalik, Tomotaroh Granzier-Nakajima, Aurelien Lherbier, Lavish Pabbi, Mauricio Terrones, Adri C Van Duin, Jean-Christophe Charlier, Eric Hudson We present a novel approach to create an effective 2D lateral heterostructure – by periodic modulation of lattice strain. We engineer extreme (>10%) strain in graphene by draping it over large Cu step edges. Analogous to a draped tablecloth, nanoscale periodic ripples arise as graphene is pinned and pulled by the contact forces of the substrate. The ripples are characterized by large variations in carbon-carbon bond length. Such variations directly impact electronic coupling between atoms, which in one graphene ripple can be as different as in two different materials. The result is a single graphene sheet which effectively acts as an electronic superlattice in which novel electronic states arise at the interfaces. Such intense, highly inhomogeneous fields create a new electronic quantization distinct from the usual Landau quantization observed in uniform fields, and their nanoscale periodicity creates a novel electronic system which can aid in the realization of various theoretical proposals including valley filters, snake states and electron optics in graphene and other 2D materials. |
Monday, March 15, 2021 3:36PM - 3:48PM Live |
C42.00004: Mechanisms of Andreev reflection in graphene in a quantum Hall regime Antonio Manesco, Ian Matthias Flór, Chun-Xiao Liu, Anton Akhmerov We explore nonlocal Andreev signatures at the zeroth Landau level of quantum Hall (QH) graphene, considering effects of (i) disorder and (ii) Fermi energy mismatch between graphene and the superconductor (SC). In both cases, our calculations show the switching between normal and Andreev reflections, caused by intervalley scattering and transition to higher Landau levels at the QH/SC interface, respectively. While the first is suppressed with high-quality interfaces, the latter cannot be canceled with gate control and must be taken into account when analyzing measurements. Our results, despite in qualitative agreement with the data, contrast with the interpretation of a recent experimental work claiming the observation of chiral Andreev states interference [Nature Physics 16, (2020)], which we showed to be strongly sensitive to the interface orientation and, in general, unlikely to happen. |
Monday, March 15, 2021 3:48PM - 4:00PM Live |
C42.00005: Quasiparticle scattering induced by native defects in graphene devices Frederic Joucken, Cristina Bena, Zhehao Ge, Vardan Kaladzhyan, Sarah Pinon, Eberth A Quezada, Takashi Taniguchi, Kenji Watanabe, Jairo Velasco Jr. Atomic-scale defects in graphene-based heterostructures have been conjectured to have dramatic effects on their transport properties. To date however, no direct evidence for the presence of defects in graphene devices made from exfoliated graphite has been reported. We report scanning tunneling microscopy results which demonstrate for the first time the presence of native defects and their effect on the electronic properties of Bernal-stacked bilayer graphene devices. Quasiparticle interference measurements at various backgate voltages show a dependence of the scattering induced by the defects on the charge carrier concentration and type. We compare our experimental results to T-matrix-based simulations and discuss the various types of defects encountered in the devices. We also present results obtained on the graphite parent crystal which demonstrate the defects are already present in the parent crystal and are not created during the making of the device. We anticipate these results will have implications for various graphene-based devices, including magic-angle twisted bilayer graphene, for which atomic scale defects have been predicted to have considerable effects on the transport properties. |
Monday, March 15, 2021 4:00PM - 4:12PM Live |
C42.00006: Dirac Point Photo-Thermionic Response at the Graphene-Boron Nitride Interface Jacky Wan, Trevor Arp, Nathaniel Monroe Gabor The unique inter- and intra-layer transport processes in graphene-boron nitride-graphene heterostructures give a unique snapshot of non-equilibrium electronic phases in this Dirac electronic system, and unveil new design principles based on the high level of control of the hot carrier distribution. Utilizing a near-infrared scanning pulsed laser, we explore the inter- and intra-layer photocurrent of a pixel consisting of two graphene layers separated by an ultrathin tunneling barrier. We find that the interlayer photocurrent increases super-linearly with excitation power, consistent with carriers being thermally driven over the interlayer barrier. The inter-layer photocurrent also exhibits a striking enhancement of the conductance near the Dirac point, which indicates a tunable electronic temperature that is sensitive to the doping and quality of each graphene layer. Furthermore, the intra-layer photocurrent is consistent with, but not fully described by, a thermoelectric current driven by a spatial temperature distribution generated from the Gaussian laser spot. The evolution of hot electronic charge carriers in Dirac electronic systems, such as graphene, may highlight the intriguing hot carrier dynamics of non-conventional electronic states, such as the Dirac fluid. |
Monday, March 15, 2021 4:12PM - 4:24PM Live |
C42.00007: Thermal transport in compensated semimetals: a mystery explained Mohammad Zarenia, Alessandro Principi, Giovanni Vignale It is well known that clean compensated semimetals, e.g. two-dimensional monolayer and bilayer graphene near the charge neutrality point, can exhibit greatly enhanced Lorenz ratio between the electronic thermal conductivity and the electric conductivity. In contrast to this, three-dimensional compensated semimetals such as WP$_2$ and Sb with indirect negative gap typically exhibit a reduced Lorenz ratio. We propose that the reason for this puzzling difference lies in the ability of indirect-gap semimetals to sustain sizeable regions of electron-hole accumulation near the contacts, which in turn is a consequence of the large separation of electron and hole pockets in momentum space. These accumulations suppress the ambipolar conduction mechanism and effectively split the system into two independent electron and hole conductors. We present a quantitative theory of the crossover from ambipolar to unipolar conduction as a function of the size of the electron-hole accumulation regions, and show that it naturally leads to a sample-size-dependent thermal conductivity. |
Monday, March 15, 2021 4:24PM - 4:36PM Live |
C42.00008: Measurement of the electronic heat capacity of graphene Aamir Mohammed Ali, John N. Moore, Xiaobo Lu, Paul Seifert, Dirk R. Englund, Kin Chung Fong, Dmitri K. Efetov Heat capacity is an invaluable quantity in condensed matter physics and yet has been completely inaccessible in two-dimensional (2D) van der Waals (vdW) materials, owing to a lack of calorimeters capable of operating at nanoscale. Here, we develop a proof-of-concept electronic calorimeter with record sensitivity (< 10-20 J/K) by combining a ~20 mK/Hz1/2–sensitive Johnson-noise thermometer with a novel optical heater operating in the low terahertz frequency domain. It measures thermal conductance Gth of nanoscale cooling pathways and temperature relaxation time τ of the system in a niche range of 0.5 picosecond to 1 nanosecond, accessing orders of magnitude faster relaxation than previous state-of-the-art calorimeters, and thereby determines the heat capacity C = τGth. We demonstrate the first ever measurement of electronic specific heat of graphene, and verify its ultra-low magnitude and proportionality to carrier density and temperature down to 15 K with a record resolution of 36 kB/μm2. This calorimetry technique can be implemented on other 2D vdW materials and devices based thereof, unlocking a new and powerful approach to their research. |
Monday, March 15, 2021 4:36PM - 4:48PM Live |
C42.00009: Probing spin and valley states in bilayer graphene quantum dots Christian Volk, Luca Banszerus, Samuel Möller, Katrin Hecker, Corinne Steiner, Eike Icking, Kenji Watanabe, Takashi Taniguchi, Christoph Stampfer Bilayer graphene (BLG) is an attractive host material for spin qubits due to its small spin-orbit and hyperfine interaction, as well as the possibility to open a gate voltage controllable band gap. The development of gate-defined quantum dots (QDs) in ultra-clean van der Waals heterostructures, where a BLG sheet is encapsulated in hexagonal boron nitride and a graphite crystal is used as a back gate, has led to a boost in device quality. Recently, the electron/hole crossover in a BLG QD [1] and the single electron occupation in a double QD [2] have been shown. |
Monday, March 15, 2021 4:48PM - 5:00PM Live |
C42.00010: Acoustic Plasmon Focusing in Graphene with an Applied Current. Michael Sammon, Dionisios Margetis, Tony Low Nonreciprocity in the plasmonic spectrum opens fundamentally new possibilities in the field of plasmonics. Here we present a theory of the nonreciprocity induced in the acoustic plasmon spectrum of a graphene-dielectric-metal system with an applied current bias. We show that the bias induces a redshift (blueshift) of the plasmonic spectrum whose wave vector is antiparallel (parallel) to the direction of the applied current. While such an effect has been discussed previously for the conventional plasmon in graphene, we show that the acoustic plasmon exhibits a focusing effect that does not occur in the conventional plasmon. We find that at large enough currents a significant portion of the plasmonic spectrum is effectively damped out, focusing the plasmon along the direction of the applied current. We argue that the source of this focusing effect is crossing of the redshifted branch into the particle-hole continuum. We present analytical and numerical results, and discuss the experimental conditions necessary to observe such an effect. |
Monday, March 15, 2021 5:00PM - 5:12PM Live |
C42.00011: Coherent Jetting behind a gate-defined Channel in Bilayer Graphene Carolin Gold, Angelika Knothe, Annika Kurzmann, Aitor Garcia-Ruiz, Kenji Watanabe, Takashi Taniguchi, Vladimir Falko, Klaus Ensslin, Thomas Ihn Using scanning gate microscopy, we image distinct electronic jets emanating from a narrow split-gate defined channel in bilayer graphene. We find that these jets, which are visible via their interference patterns, occur predominantly with an angle of 60° between each other. This observation is related to the specific bandstructure of bilayer graphene, in particular trigonal warping, which leads to a valley-dependent selection of momenta for low-energy conduction channels. This experimental observation of electron jetting has consequences for carrier transport in graphene in general as well as for devices relying on ballistic and valley selective transport. Quantum devices utilizing the valley degree of freedom will need to consider and possibly exploit this fundamental effect of the trigonally warped bandstructure of bilayer graphene. |
Monday, March 15, 2021 5:12PM - 5:24PM Live |
C42.00012: Electrostatic Tuning of Correlations in Graphene Nicholas Dale, Ryo Mori, Iqbal Bakti Utama, Jonathan Denlinger, Conrad Stansbury, Sihan Zhao, Kyunghoon Lee, Takashi Taniguchi, Kenji Watanabe, Eli Rotenberg, Roland Koch, Aaron Bostwick, Chris Jozwiak, feng wang, Alessandra Lanzara The Landau-Fermi Liquid Theory maps an interacting liquid of electrons to a non-interacting gas of quasiparticles. This picture breaks down in Graphene, because the bare Coulomb interaction is preserved near the charge neutrality point. In this talk, I will discuss recent in-operando angle-resolved photoemission studies on graphene where we directly visualize modifications of its electronic band structure upon tuning the Fermi Energy. |
Monday, March 15, 2021 5:24PM - 5:36PM On Demand |
C42.00013: Open quantum dot in encapsulated graphene Kohei Sakanashi, Naoto Wada, Kenji Watanabe, Takashi Taniguchi, Gil-ho Kim, David K Ferry, Jonathan P Bird, Liang Huang, Nobuyuki Aoki Graphene open quantum dot (GOQD) which is the small cavity strongly connected to reservoir is one of candidate device system, to observe carrier dynamics and interference phenomena inside the cavity with spin and valley information. |
Monday, March 15, 2021 5:36PM - 5:48PM On Demand |
C42.00014: Sizable Band Gap in Epitaxial Bilayer Graphene Induced by Silicene Intercalation Hui Guo, Ruizi Zhang, Hang Li, Hong Ding, Shixuan Du, Sokrates T Pantelides, Hongjun Gao Absence of a band gap in monolayer graphene is a big obstacle to its utilization in silicon electronics. With one more graphene layer added, however, bilayer graphene (BLG) provides a non-zero band gap by breaking the inversion symmetry. Opening a sizable band gap in BLG is of significance for potential applications in graphene-based electronic and photonic devices. Here, we report the generation of a sizable band gap in BLG by intercalating silicene between BLG and Ru substrate. We first grow high-quality Bernal-stacked BLG on Ru(0001) and then intercalate silicene to the interface between the BLG and Ru, which is confirmed by low-energy electron diffraction and scanning tunneling microscopy. Raman spectroscopy shows that the G and 2D peaks of the intercalated BLG are restored to the freestanding-BLG features. Angle-resolved photoelectron spectroscopy measurements show that a band gap of about 0.2 eV opens in the BLG. Density functional theory calculations indicate that the large-gap opening results from a cooperative contribution of the doping and rippling/strain in the BLG. This work provides insightful understanding on the mechanism of band-gap opening in BLG and enhances the potential of graphene-based device development. |
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